Reverse osmosis (RO) is an advanced membrane process used to produce potable water from a variety of water sources, including surface water, seawater, and wastewater. The yield of the product water from the RO systems is increased by the addition of antiscalants which prevent the membrane from the scaling of calcium and other ions. Removal of antiscalants from the RO concentrate can induce the precipitation of oversaturated scale-forming substances, enable additional water recovery from RO concentrates, and reduce the risk of eutrophication after concentrate disposal into the receiving water (e.g., river water). Furthermore, the oxidative treatment of RO concentrates reduces the discharge of micropollutants into the environment.

The aim of this study was to provide a better insight into oxidation reactions of the
N-free phosphonate antiscalants 1‑hydroxy ethane-1,1-phosphonic acid (HEDP) and 2‑phosphonobutane-1,2,4-tricarboxylic acid (PBTC) and of the N-containing phosphonate antiscalants amino trimethylene phosphonic acid (NTMP/ATMP) and Diethylenetriamine penta(methylene phosphonic acid) (DTPMP) regarding reaction kinetics with ozone, hydroxyl radicals (^•OH) and sulfate radicals (SO₄^•-).

In addition, the efficiency of eliminating pharmaceuticals, corrosion inhibitors, x-ray contrast media, and perfluorinated compounds from RO concentrates during ozonation and UV/persulfate processes was investigated. Second-order rate constants for the reactions of candesartan, irbesartan, methyl-benzotriazole, and chloro-benzotriazole with SO₄^•- were determined for the first time. Besides kinetic experiments, the ozone exposure and hydroxyl radical exposures, and degradation rate in different water matrices and real RO concentrates were investigated. Additionally, the by-product formation was determined and the energy demand for antiscalant elimination with the process of ozonation and UV/persulfate was calculated.

The kinetic results showed that HEDP and PBTC are ozone-refractory at pH 7, while NTMP and DTPMP are highly reactive with ozone. The second-order reaction rate constants with ^•OH and SO₄^•-  are in the typical range for organic compounds and showed no pH dependency.

The matrix degradation experiments evinced an increased HEDP degradation rate in sulfate, silicate, and chloride matrices during ozonation. Whereas carbonate and chloride hindered PBTC ozonation, and NOM inhibited both HEDP and PBTC degradation through scavenging of ^•OH. Due to their high reaction rates with ozone the ozonation of NTMP and DTPMP was not affected by inorganic water constituents. Only the addition of NOM reduced the degradation to a small extent. The SO₄^•- radical-based oxidation process of the antiscalants was mainly inhibited by bicarbonate, chloride, and NOM. The highest inhibitory effect of bicarbonate on this AOP by scavenging of SO₄^•- and ^•OH was observed for N-free organophosphonates.

This study has shown that ozonation and UV/persulfate oxidation are promising tools to remove organophosphonates from RO concentrates. HEDP could be degraded up to 60% with ozone and UV/persulfate application, whereas the oxidation of PBTC is limited to 10% degradation for both processes. NTMP and DTPMP ozonation showed the same extent of degradation in the concentrates as compared to the buffered experiments. DTPMP degradation with UV/persulfate was around 45%, whereas NTMP could be degraded up to 65% at a fluence of about 65 kJ m^-2.

UV/persulfate eliminated all micropollutants to a higher extent than ozonation in RO concentrates due to the higher yield of oxidative species and photolytic degradation. Compounds with electron-rich moieties like carbamazepine, diclofenac, metoprolol, and sulfamethoxazole were eliminated with small ozone doses (< 0.5 mg O₃ / mg DOC) and with a small fluence (< 5000 J m­^-2) in UV/persulfate processes. Photosensitive compounds with high reactivity towards ^•OH and SO₄^•- like the x-ray contrast media iopamidol, iohexol, and amidotrizoic acid were successfully eliminated with small fluence in UV/persulfate, whereas these compounds persist in ozonation at common ozone dosages.

Comparing the application of both oxidative processes to the RO concentrate, ozonation has the disadvantage of higher formation of by-products like bromate. In the presence of N-containing organophosphonates the formation of bromate is decreased due to enhanced ozone consumption. The energy demand of both processes strongly depends on the target compounds to be eliminated. For the elimination of compounds such as HEDP, NTMP, DTPMP,and sulfamethoxazole, ozonation is an economical technique, whereas UV/persulfate is better suited for the elimination of ozone-refractory compounds such as x-ray contrast media. In general, oxidative process treatment of RO concentrates could be applied to partly abate micropollutants before discharge.